The present invention relates to a method of applying a coating by physical vapour deposition onto an article of organic material, in particular nonconductive organic material such as plastic and/or epoxy material, especially ABS (Acrylonitrile-Butadiene-Styrene Copolymers) and polymers with high temperature resistance, or onto an article of another composition coated with such organic material. Furthermore, the present invention relates to an article of organic material having a coating applied by a physical vapour deposition process.
It is known to coat plastic articles by physical vapour deposition, for example to produce decorative coatings of specific colours and coatings which are resistant to wear and scratching. Such coated articles may for example be items of architectural hardware, such as door hardware, i.e. handles, finger plates etc., or also elements of taps and other water conducting fittings. Such coating processes have in the past mostly been carried out with an interlayer of nickel.
In addition to the coating of plastic articles by physical vapour deposition, it is also possible to consider providing metal articles with an organic coating, for example alloy wheels with an epoxy coating, with a further coating then being deposited by a physical vapour deposition process.
Besides the non-conductive character, another major problem of applying a coating by physical vapour deposition to plastic/epoxy articles arises in the degassing of the plastic or plastic coating. Water vapour tends to be absorbed in the organic material and cannot be degassed completely by a heat treatment. This means that degassing must be carried out under vacuum environment conditions in the PVD coating apparatus. The degassing requirements significantly restrict the productivity of the PVD apparatus and substantially increase the cost of the process.
The object of the present invention is to provide a remedy for this problem, so that extensive degassing can be minimized, so that the PVD coating process can be carried out economically and so that long-term problems associated with adsorbed water vapour in the plastic coating are avoided.
In order to satisfy this object, there is provided, in accordance with the present invention, a method of the initially named kind which is characterized in that a Cr layer is deposited on the article prior to deposition of said coating to form a diffusion barrier for water coming from said article, said Cr layer itself being deposited by a physical vapour deposition process comprising at least the following steps:
a) depositing a first layer of Cr on said article in a physical vapour deposition apparatus using an arc evaporator or a cathode sputtering source, or a combined arc evaporator and cathode sputtering apparatus, using zero bias, and
b) subsequently applying a negative bias voltage to said article having said first layer of Cr and depositing a second layer of Cr on said first layer using Cr ions in the same physical vapour deposition apparatus, with the negative bias voltage preferably being a moderate bias voltage, e.g. in the range from 10 to 50 V.
Preferably the step a) is followed by a treatment step in the same apparatus in which an elevated negative bias voltage higher than that of step b) and typically in the range from 200 V to 700 V, especially about 500 V is applied to the first Cr layer and said first Cr layer is bombarded with inert gas ions, such as argon, or metal ions generated by the apparatus, typically in the arc evaporation mode. This produces at least one of the following effects:
plasma activation of the free surface of the first Cr layer,
etching of the free surface of the first Cr layer,
implantation of Cr ions into the first Cr layer
implantation of Cr ions into a surface layer of the article beneath said first Cr layer and
densification of said first Cr layer.
Thus, according to the present invention, at least one layer of Cr is deposited on the plastic article.
It has namely been found that the Cr layer oxidises in use and in doing so forms a very stable layer which acts as a diffusion barrier for water vapour present in the substrate. Thus, rather than trying to ensure full degassing of the article prior to coating, the present invention proposes an alternative course of action in which water vapour migration from the article is limited by the Cr layer and by the very stable chromium oxide layer which arises.
The chromium oxide layer can arise in different ways. First of all, the conditions in the physical vapour deposition apparatus can lead to some of the oxygen contained in the water adsorbed in the article reacting with the chromium to form chromium oxide. Secondly, the coatings deposited on the Cr layer are still porous to oxygen to some degree, so that oxygen from the atmosphere can penetrate the coating provided on the Cr layer and react with the Cr to form the chromium oxide.
It is a particular advantage of the chromium sealing layer that oxygen does not just react with Cr at the grain boundaries but rather reacts with it over the full surface.
As can be appreciated from the foregoing, the Cr layer is preferably deposited in several steps. Articles consisting of organic materials such as plastics and polymers are typically non-conducting, so that it is not possible to apply a bias voltage to them in an attempt to attract charged chromium ions onto the surface of the article. However, the present invention recognises that it is nevertheless possible to deposit Cr onto such a non-conducting article, or onto an article having a non-conductive surfacexe2x80x94such as an epoxy-coated metal wheelxe2x80x94in a physical vapour deposition apparatus, because the conditions prevailing in a physical vapour deposition apparatus during coating means that some of the Cr ions will become neutral, will reach the surface of the article because of kinetic considerations and will be deposited there.
Once a thin layer of Cr has been deposited in this way, the article will have a conductive surface and it is then possible to apply a bias voltage to the article, i.e. to the first Cr layer. The article is held by a metallic fixture and the thin chromium layer deposited on the article forms a bridge to this metallic fixture, so that a bias voltage applied to the metallic fixture is automatically applied to chromium layer on the article by the chromium bridge. This voltage can then be used to promote the attraction of ions so that the article is subjected to ion bombardment. The initial ion bombardment is preferably carried at a relatively high bias voltage, typically in the range from 200 V to 700 V, for example 500 V, in order to ensure adequate ion treatment of the surface.
This ion bombardment has several effects. On the one hand, it produces ion etching of the first layer of Cr, which promotes the bonding of ions subsequently deposited on and coating the surface of the first layer. Secondly, the ion bombardment results in chromium ions being deposited within the first layer and, provided sufficient bias voltage is present, in some chromium ions penetrating the first layer completely and becoming lodged in the surface layer of the article, i.e. in the organic material of the article. These implantation mechanisms promote the adhesion of the chromium layer to the article and also serve to densify the first chromium layer.
Furthermore, even if Cr ions are not implanted into the substrate material during ion bombardment, the plasma present in the physical vapour deposition apparatus serves to activate the substrate surface to promote subsequent coating.
Once the surface of the article has been etched and activated in this manner, chromium ion deposition is continued, but at a more moderate bias voltage of typically 10 to 50 V, especially about 30 V, so that a further layer of Cr is deposited on the surface of the article. The deposition of this further Cr layer at moderate bias results in a denser Cr layer with a better barrier capacity for water and water vapour.
Whereas the first layer of Cr typically has a thickness of up to about 0.1 xcexcm max, the further layer is conveniently executed with a thickness in the range 0.1 xcexcm to 0.4 xcexcm, with 0.2 xcexcm being considered sufficient for most purposes.
Once the total Cr coating has been performed in this way, the actual coating required can be deposited. It is possible to deposit this coating straight onto the further Cr layer, using known physical vapour deposition techniques. It is, however, frequently better to first apply a CrN interface layer to improve the adhesion between the chromium and the top coatings, in particular ceramic top coatings, which are subsequently applied.
Once the CrN interface layer has been deposited, or after deposition of the further Cr layer if no interface layer is provided, the actual desired coating is then deposited on the article, for example to give the required colour and/or abrasion resistance of the surface. Typical coatings are, for example, applied in the form of a stack of coatings comprising alternating layer sequences of various materials. Coatings which are of particular interest comprise coatings selected from the group comprising CrNxe2x80x94CrC, CrNxe2x80x94CrZrNxe2x80x94ZrN and CrNxe2x80x94CrCxe2x80x94ZrCN. However, other coatings may also be applied, such as TiN or TIAlN.
Coatings containing nitrogen are most conveniently deposited by introducing nitrogen into the atmosphere of the physical vapour deposition apparatus. Coatings involving carbon can either be produced from graphite cathodes or by introducing suitable organic gases into the atmosphere of the physical vapour deposition apparatus, such as C2H2.
Further preferred embodiments of the method and of articles coated in accordance with the method are set forth in the claims appended hereto.
The invention will now be described in more detail with reference to a preferred embodiment and to the accompanying drawings.